Glucocorticoids are a class of steroid hormones characterized by an ability to bind with the cortisol receptor and trigger similar effects. Glucocorticoids have potent anti-inflammatory and immunosuppressive properties and as such are the best-known class of anti-inflammatory active ingredients. Owing to their broad range of uses and their great anti-inflammatory action, corticoid preparations are therapeutic agents of first choice in a wide variety of inflammatory diseases, such as, for example, diseases of the rheumatoid group, allergies, inflammatory diseases of the lungs, heart, and intestines, bronchial asthma, hyperproliferative diseases of the skin (psoriasis), eczemas, auto-immune diseases, or states of shock.
Dexamethasone, a synthetic member of the glucocorticoid class of hormones which exhibits anti-inflammatory and immunosuppressive properties which are 40 times more potent than naturally-occurring hydrocortisone and having the following structure,
9-fluoro-11β,17,21-trihydroxy-16a-methylpregna-1,4-diene-3,20-dione, has been shown to augment the antiemetic effect of 5-HT3 receptor antagonists. Dexamethasone is used to treat many inflammatory and autoimmune conditions, such as rheumatoid arthritis. It is also given to cancer patients undergoing chemotherapy to counteract certain side-effects.
Glucocorticoids, particularly dexamethasone, have been used for years to treat preterm infants who have or are at risk for chronic lung disease (The, T. F. et al., Early Postnatal Dexamethasone Therapy for the Prevention of Chronic Lung Disease in Preterm Infants with Respiratory Distress Syndrome: A Multicenter Clinical Trial, 1997, 100:715-6). These agents often have the short-term benefits of improving lung compliance and facilitating early weaning from mechanical ventilation. Newborns, with extremely low birth weight (ELBW), traditionally receive early postnatal dexamethasone therapy to treat and/or prevent severe respiratory distress syndrome and the subsequent onset of chronic lung disease.
Glucocorticoids, however, suppress growth hormone secretion. Human growth hormone (also may be referred to herein as “GH”) is a single-chain-polypeptide consisting of 191 amino acids (molecular weight 21,500). Disulfide bonds link positions 53 and 165 and positions 182 and 189 (Niall, Nature, New Biology, (1971), 230: 90). Effects of growth hormone on the tissues of the body can generally be described as anabolic (building up). Like most other protein hormones, GH acts by interacting with a specific receptor on the surface of cells. Height growth in childhood is the best known effect of GH action. GH also stimulates production of insulin-like growth factor 1 (may also be referred to herein as “IGF-1”) which demonstrates growth-stimulating effects on a wide variety of tissues. GH also serves many other metabolic functions such as increasing calcium retention and mineralization in bones, increasing muscle mass by inducing protein synthesis, stimulating the immune system, reducing liver uptake of glucose thus contributing to the maintenance and function of pancreatic islets and promoting lipolysis, which results in some reduction of adipose tissue (body fat) and rising amounts of free fatty acids and glycerol in the blood.
The pulsatile release of growth hormone from the pituitary somatotrops is regulated by two hypothalamic neuropeptides: growth hormone-releasing hormone and somatostatin. Growth hormone-releasing hormone stimulates release of growth hormone whereas somatostatin inhibits secretion of growth hormone (Frohman et al., Endocrinology Review (1986) 7:223-53 and Strobi et al., Pharmacology Review (1994) 46:1-34). Most GH deficiencies are caused by defects in GH release, not primary defects in pituitary synthesis of the hormone itself. Increasing GH secretion can be achieved by stimulating or inhibiting various neurotransmitter systems in the brain and hypothalamus. As a result, the development of synthetic growth hormone-releasing agents to stimulate pituitary GH secretion is being pursued and may have several advantages over expensive and inconvenient GH replacement therapy. By acting along physiologic regulatory pathways, the most desirable agents would stimulate pulsatile GH secretion, and excessive levels of GH that have been associated with the undesirable side effects of exogenous GH administration would be avoided by virtue of intact negative feedback loops.
It is hypothesized that the pathogenesis of glucocorticoid mediated growth inhibition is most likely multifactorial in nature involving, partial growth hormone resistance, suppression of IGF-1 activity, and antagonism of insulin activity. These factors all influence carbohydrate and lipid metabolism.
By inhibiting GH secretion, elevated glucocorticoid levels can induce protein catabolism which in turn can lead to the degradation of skeletal muscle or the atrophy of intestinal villi. Deficiency in growth hormone results in a variety of medical disorders. The consequences of acquired GH deficiency include profound reduction in lean body mass and concomitant increase in total body fat, particularly in the truncal region. Decreased skeletal and cardiac muscle mass and muscle strength lead to a significant reduction in exercise capacity. Bone density is also reduced.
Concern has been expressed regarding the effects of early dexamethasone therapy on somatic growth because glucocorticoids have been shown to alter cell size and DNA synthesis in animal models (Cotterrell M. et al., Effects of Corticosteroids on the Biochemical Maturation of Rat Brain: Postnatal Cell Formation, J. Neurochem., 1972, 19:2151-67). In addition, it has been found that dexamethasone therapy may compromise the accretion of bone mineral and this affect the velocity of bone growth, even when energy intake increases (Weiler, H. A. et al., Longitudinal Assessment of Growth and Bone Mineral Accretion in Prematurely Born Infants Treated for Chronic Lung Disease with Dexamethasone, Early Hum. Dev., 1997, 47:271-86 and Gibson, A. T. et al., Growth Retardation After Dexamethasone Administration: Assessment by Knemometry, Arch. Dis. Child, 1993, 69:505-9).
As reported in the New England Journal of Medicine, children who received early postnatal dexamethasone therapy for severe respiratory distress of prematurity were observed to have more neuromotor and cognitive function impairment and disability at school age than premature children not treated with dexamethasone. A study conducted at the China Medical University, Taichung, Taiwan, revealed that children treated with dexamethasone had significantly smaller head circumference and significantly lower mean height. In addition, dexamethasone-treated children were observed to evidence significantly poorer motor skills and motor coordination as well as poorer visual-motor integration as compared to children not treated with dexamethasone. The observed increase in neurodevelopmental dysfunction in neonates treated with dexamethasone led the Taiwanese researchers to recommend the discontinuation of use of a dexamethasone regimen to chronic lung disease in children despite its benefits due to its adverse effects on somatic growth at school age (Hendry, J., Postnatal Dexamethasone Treatment Associate with Later Neuromotor and Cognitive Function Impairment and Disability, 2004, N. England J. Med., 350:1304-13).
Glucocorticoid therapy is considered essential to the management of asthma; such treatments are often given on a daily basis and for an extended period of time. Recent studies have shown that glucocorticoid administration, while alleviating some symptoms of asthma, may also lead to airway damage or airway remodeling (see Dorscheid, D. R. et al., “Apoptosis of airway epithelial cells induced by corticosteroids”, Am. J. Respir. Crit. Care Med., 2001, 164:1939-1947). As noted by Dorscheid, treatment of asthma with corticosteroids such as dexamethasone and the resulting induction of cell death “raises the possibility that at least one of the major components of chronic airway damage in asthma, epithelial shedding and denudation, may in part result from a major therapy for the disease.”
Patients who must take large doses of pharmacological glucocorticoids, such as dexamethasone, can develop Cushing's syndrome if exposed to high enough doses over an extended period of time. Cushing's syndrome is a condition which is associated with a number of negative catabolic effects, including reduced growth velocity and lean body mass. A person suffering from Cushing's syndrome usually has a large, round face (commonly referred to as a “moon face”) with slender arms and legs in proportion to the thickened trunk. The catabolic effects of this disease results in limited muscle capacity which leads to pronounced physical weakness. The skin becomes thin, bruises easily and heals poorly when bruised or cut. The heightened glucocorticoid levels associated with Cushing's syndrome, over time, results in chronic, elevated blood pressure, osteoporosis, diminished resistance to infections, the development of kidney stones and diabetes. Mental disturbances, including depression and hallucinations, have been found to occur in persons having Cushing's syndrome. Women usually experience irregular menstrual cycles. Children with Cushing's syndrome grow slowly and remain short. In some people, the adrenal glands also produce large amounts of androgens. Chronic glucocorticoid excess associated with Cushing's syndrome, left untreated, increases the risk of premature death.
Ghrelin likely enhances the activity of growth hormone releasing hormone (GHRH)-secreting neurons while concomitantly acting as a functional somatostatin (SS) antagonist (Ghigo, E. et al., Eur J Endocrinol (1997) 136(5):445-60). The observed ability of ghrelin to enhance food intake, increase food assimilation and gastric emptying, together with its ability to increase GH levels, thus promoting prompt nutrient incorporation into muscle and fat reserves, indicates that ghrelin may have therapeutic potential to treat indications wherein protein catabolism is a symptom.
Long-term administration of glucocorticoids is one of the most used treatments in clinical medicine but is known to suppress GH secretion and action. In fact, glucocorticoids inhibit pulsatile GH release, reduce GH receptor expression and signal transduction and inhibit IGF-1 bioactivity. Recognition of glucocorticoid-mediated antagonism of GH secretion and action has renewed interest in GH therapy or treatments to stimulate GH release as a potential means to reverse some of the most harmful side effects of glucocorticoid long-term treatment, such as growth inhibition and catabolic effects. Different studies suggest that the detrimental effects of glucocorticoid can be variably overcome by GH treatment, but long-term GH therapy also has the potential for adverse effects and requires further surveillance and study. It has been extensively demonstrated in humans and animals that the inhibitory effects of long-term glucocorticoid administration on GH secretion are mediated by increased somatostatin tone. Synthetic GHS appear to stimulate GH release, in part, through inhibition of somatostatin pathway; indeed, GHRP-6 is able to completely counteract glucocorticoid-mediated GH-inhibition in the rat.
It is a primary objective in the art to maximize the beneficial effects of glucocorticoids, in particular dexamethasone, while minimizing their adverse effects. The catabolic side effects of glucocorticoids prevent this class of substances from being put to an even broader range of therapeutic uses. Despite the reduced side-effect potential of modern corticoids, especially long-term treatment with active ingredients of this class of substances remains critical. Thus, there exists in the art the need for agents and methods to counteract the negative effects of and, thus to enhance the beneficial effects of, the long term administration of glucocorticoids.